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Abstract
For the ViewRay® system (ViewRay Inc., Cleveland, OH, USA) which is representative of magnetic resonance (MR) guided radiotherapy machine, it is important to evaluate effectiveness of AAPM's TG-51 protocol and the effect of the magnetic field on absolute dosimetry. In order to measure the absolute dose, MR-compatible chamber and water phantom system manufactured in this study were used. The materials of the water phantom system were plastic of polymethyl methacrylate (PMMA) and non-ferrous materials. Due to the inherent feature of the ViewRay®, all Co-60 sources are not located at gantry angle of 0° while being located at gantry angle of 90°. For this reason, absolute dosimetry was performed based on the measurements in solid water phantom (SWP) and water which determine the SWP to water correction factor. For evaluation of output constancy with gantry angle, measurements were made with ionization chamber inserted in cylindrical water-equivalent phantom. For measured doses in water, the values of dose deviation according to a reference dose of 200 cGy for Head 1, Head 2 and Head 3 were -0.27%, -0.45% and -0.22%, respectively. For measured doses in SWP, the values of dose deviation according to a reference dose of 200 cGy for Head 1, Head 2 and Head 3 were -1.91%, -2.07% and -1.84%, respectively. All values of dose measured in SWP tended to be less than those measured in water by -1.63%. With the reference gantry angles of 0° and 90°, the maximum values of deviation for Head 1, Head 2 and Head 3 were 0.48%, 1.06% and 0.40%, respectively. The measurement agreement is within the range of results obtainable for conventional treatment machines. The low strength of the magnetic field does not affect dose measurements. Using the SWP to water correction factor, absolute doses for ViewRay® system can be measured.
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Fig. 1
Water phantom system without ferrous materials was manufactured. Water phantom system consists of (a) water phantom tank, (b) plastic ruler, and (c) thumbscrew type moving system.
Fig. 2
Dose deviations between measured absolute doses of all Co-60 sources and a reference dose of 100 cGy. After ViewRay® system was commissioned on July 2016, absolute doses were measured for every month.
Table 1.
Correction factors of AAPM's TG-51 and absolute dose values in water and solid water phantom (SWP) for Head 1, Head 2, and Head 3. Measurements in water were performed to deliver 200 cGy to reference point when both Head 1 and Head 3 were set to gantry angle of 0°. Measurements in SWP were performed to deliver 200 cGy to reference point when Head 1, Head 2 and Head 3 were set to gantry angle of 90°.
| Correction factor | Output | ||||||
|---|---|---|---|---|---|---|---|
| Pion | Pelec | Ppol | ∗Dw, 0° (cGy) | †Dev200 cGy (%) | ‡DSWP, 90° (cGy) | Dev200 cGy (%) | |
| Head 1 | 1.001 | 1.000 | 0.999 | 199.5 | −0.27 | 196.2 | −1.91 |
| Head 2 | §199.1 | −0.45 | 195.9 | −2.07 | |||
| Head 3 | 199.6 | −0.22 | 196.3 | −1.84 | |||
Table 2.
Output constancy with gantry angle. The reference measurements were gantry angle of 0° for Head 1 and Head 3, and 90° for Head 2.
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